(1) Field of the Invention
The invention is related to an empennage/fairing structure of a helicopter with a ducted counter-torque device with the features of the preamble of claim 1.
(2) Description of Related Art
Shrouded anti-torque systems for helicopters consist essentially of an empennage structure including a shroud and a ducted fan with an airflow duct and a fan assembly coaxially mounted within the airflow duct. It is a fact, that all anti-Torque Systems for helicopters produce noise additional to the noise of the main rotor. While conventional rear rotors for helicopters are often complex, fragile, and require large rotor diameters, the faired/shrouded anti-torque rotors are smaller because of better aerodynamic efficiency and they eliminate the risk of accidents due to the protection from impacts provided by the fairing.
The fan assembly includes a central hub structure, a number of support struts for mounting of the hub structure in the airflow duct. The support struts usually are elliptical shaped in order to enhance the aerodynamic performance and they may be configured as stators for flow-straightening and thus recovery of rotational energy from the airflow.
The acoustic energy which is emitted from these devices is essentially dependent on the flowing out processes, overflow phenomena of bodies and rotating pressure fields. For shrouded tail-rotors there might be additional sources of noise due to body resonances and cavity effects. While the fan noise is shielded by the nacelle of the helicopter in flight direction, non-optimized shrouded tail-rotors emit still significant amounts of noise to the back and to the sides of the helicopter.
A major source for acoustic energy emitted from shrouded/ducted tail-rotors can be the turbulences between the blade tips and the shroud. The larger the distance between the blade tips and the shroud, the more acoustic energy is produced by these turbulences. This so-called “Clearance Noise” is commonly known as a moderate, but broad-band increase of the noise level. Frank Kameier in “Experimentelle Untersuchung zur Entstehung and Minderung des Blattspitzen-Wirbellärms axialer Strömungsmaschinen. Diss. TU Berlin, Hermann-Föttinger-Institut für Thermo-und Fluiddynamik. Berlin 1994” has shown that there are significant increases of noise levels for specific narrow frequency ranges, if the clearance is more than 0.003 times the rotor diameter.
Various methods have been proposed in the past to reduce the level of the annoying and disturbing noise of the anti-torque devices:                1. The documents U.S. Pat. No. 5,588,618; EP680873 respectively disclose aerodynamic optimization of the support struts and stators in order to reduce the separation of vortices;        2. The documents EP 562527; EP680871 respectively disclose unequal angular spacing of the rotor blades in order to distribute the acoustic noise over a greater number of fundamental frequencies; and        3. The documents U.S. Pat. Nos. 5,634,611; 8,061,962 respectively disclose harmonization of the number and arrangement of rotor blades and stators in order to reduce the interferences between the blades and the stators.        
Due to the increase of the number of fundamental frequencies and their harmonics, the noise of the tail structure of the helicopter is a kind of broad band noise which is able to excite a lot of body and cavity resonances. In fact, the tail structure of the helicopter, especially the empennage has turned into a kind of “musical instrument” like a guitar, which is amplifying a broad spectrum of the remaining audible sound.
The document WO 2010 118860 discloses composite components and heat-curing resins and elastomers and relates to a plastic composite component which is formed by a thin hard plastic outer layer, at least one elastomer layer adjoining the former on the inside, and at least one metal and/or plastic carrier layer adjoining said elastomer layer on the inside and made of a fibre reinforced plastic (carbon or glass fibre). This should act among other as an impact protection part, as a splinter protection part or as a protective part against vibrations and vibration damages, against resonance, for the purpose of damping oscillations or for the purpose of acoustic damping of amongst others rotor blades and aircrafts parts.
The document EP2071561 discloses an absorbent structure for rotor noise and a rotor duct. The structure has a separation unit for arranging a porous wall at fixed distance from a rigid baffle made of glass fiber by defining cavities with a height between the porous wall and the baffle, where the height is determined to obtain maximum absorption of acoustic waves emitted at basic frequency. An additional porous wall is arranged in the cavities at intermediate height to obtain maximum absorption for another basic frequency. The porous walls have an absorbent layer made of fine-mesh fence and another absorbent layer made of fiber felt. The application of Helmholtz-Resonators in order to absorb noise and to reduce the sound level of the shrouded tail rotor is suitable to reduce the amplitudes of specific frequencies, i.e. the frequencies for which the resonators are tuned as well as for their harmonics. However it is very difficult—if not impossible—to completely reduce the broad-band noise with stochastic frequencies as it results from the operation of the shrouded tail rotor.
The document US2009014581 A1 discloses a ducted fan for a helicopter with a transverse duct and a counter-torque device supported within the duct. The counter-torque device includes a rotor rotatably mounted within the duct and a stator fixedly mounted within the duct downstream from the rotor. The rotor includes a rotor hub having a rotor axis, and rotor blades extending from the hub. The Rotor blades have a modulated angular distribution about the rotor axis. The stator includes a stator hub, and a plurality of stator vanes distributed around the stator hub. The stator vanes are angularly modulated around the stator hub.
The document U.S. Pat. No. 6,206,136 B1 discloses an absorptive acoustic liner in which the perforated facesheet has a coating of an erosion-resistant material applied thereto. The coating can be applied using a simple spraying process. The acoustic liner can be easily tuned by regulating the thickness of the coating.